The use of tin-lead based solders for electronic packaging is well established. Market and legislative pressures are accelerating the reduction, and perhaps elimination, of lead from solders used in electronics packaging. Unfortunately, lead-free alloys typically have poorer wetting characteristics than lead based solder alloys.
Tin-lead solder is widely used in the electronics industry for electrical and mechanical interconnections between devices and chip carriers and between chip carriers and printed wiring boards. The interconnection between a chip carrier and a printed wiring board often involves a ball grid array (BGA) interconnection.
The manufacture of a BGA interconnection on a chip carrier usually involves the attachment of solder members, for example, spheres, to conductive (and solderable) pads on the chip carrier. Solder paste is another method used to form the BGA interconnection. Regardless of the process used, a solder enhancing material such as a solder flux is usually used to enhance solderability. Solder flux serves two important functions. First, it provides an adhesive property that maintains solder members in position after they are positioned on conductive pads until a mechanical interconnection is made by soldering. Flux also functions to remove oxides from the metallic surfaces of both the solder members and conductive pads, promoting a robust metallurgical bond, during reflow and solidification, between solder members and conductive pads.
Solder fluxes used in the electronics industry vary depending on application method and whether the flux is water soluble or no-clean. A water soluble flux is a flux that leaves aqueous residue behind that can be cleaned with water after processing. A no-clean flux is a flux that leaves non-ionic residues behind that are typically left on the parts for further processing. Either type of flux can be sprayed, screened, or transferred by pins onto the conductive pads to be soldered. Sometimes solder members are dipped into flux and then positioned on conductive pads.
During the heat up and reflow process used to make the interconnection, flux chemically reacts with oxides on the surface of the solder members/conductive pads and oxygen in the reflow oven environment. Increasing the quantity of flux used, usually improves the solder wetting but can negatively affect the process yield because the solder members may be able to “float” up and off the conductive pads and/or create electrical bridges (shorts) with neighboring interconnections comprising the BGA.
Typically, tin-lead solders exhibit robust wetting in air reflow ovens, however the wetting of lead-free solders is negatively impacted especially by the presence of oxygen during reflow. Substitution of nitrogen for air in the lead-free reflow process can be very expensive and cumbersome.
As a result, there exists in the industry a need for a more reliable, less costly method of making an interconnection between a solder member, particularly a lead-free solder member, and a conductive pad of a substrate to make a circuitized substrate which overcomes the disadvantages of the known method and structure.